cylinder

A cylinder is the precisely machined metal bore in which a piston slides up and down, driven by combustion pressure in an engine or by external force in a pump or compressor. The cylinder walls must be smooth and round to within a few thousandths of an inch, because any deviation causes excess friction, blow-by of gases, and premature wear. A typical automotive gasoline engine has between 3 and 8 cylinders, while heavy diesel engines may have 12 or more.

The cylinder is cast or forged as part of the engine block, and its interior surface, the bore, is finished by honing, a process that leaves a controlled crosshatch pattern to help retain oil on the piston rings. Bore diameter is a fundamental engine parameter; a 4-liter engine with 8 cylinders has an average bore size around 91 millimeters. The length of the bore relative to the stroke of the piston determines whether an engine is oversquare (bore larger than stroke, allowing higher rpm) or undersquare (bore smaller than stroke, favoring torque).

Cylinder wear happens gradually over time as the piston rings scrub the walls. Scoring, caused by broken ring fragments or abrasive particles in the oil, creates visible grooves that allow compression and combustion gases to escape past the rings. Glaze, a smooth hard layer that forms on the bore wall, reduces oil retention and accelerates ring wear. Both defects eventually require cylinder reboring to the next oversize standard, typically 0.010, 0.020, 0.030 inch, or metric equivalents, and fitting oversized pistons and rings.

Wet cylinder sleeves are replaceable steel tubes pressed or bolted into the engine block and in direct contact with the coolant jacket; they allow a damaged bore to be restored by simple sleeve replacement rather than a costly block rebore. Dry sleeves sit between the bore and the coolant and are rarely removed during engine life. Both types are standard in heavy truck and industrial diesel engines, where cost of downtime justifies the precision required for installation.

The term cylinder itself comes from the Greek kylindros, meaning roller or cylinder, which describes the geometry but reveals nothing of its function under extreme conditions. In a running engine, cylinder temperatures exceed 2000 degrees Fahrenheit in the combustion zone, while coolant jackets surrounding the bore try to maintain metal temperature around 200 to 220 degrees. This thermal stress, combined with the mechanical shock of thousands of explosions per minute, makes cylinder integrity fundamental to engine longevity and reliability.